The present invention relates to a semiconductor unit of the type wherein two semiconductor rectifier devices are electrically insulated and thermally conductively arranged in an electrical series circuit so that a current conducting lead associated with the input, a current conducting lead associated with the output of the series circuit as well as a third current conducting lead associated with the connecting conductor between the two semiconductor devices are arranged in a row and each one of the two outer current conducting leads, together with its respective associated semiconductor device, is firmly mounted to a respective contacting plate which is fastened on a base plate via a wafer of electrically insulating, thermally conductive material, and wherein the semiconductor devices have their contacting and connecting members accommodated in a housing including the base plate and a hollow plastic member fastened to the base plate.
Such semiconductor units are known from the above-identified copending U.S. patent application, Ser. No. 677,851, the subject matter of which is incorporated herein by reference. The semiconductor units have a compact structure and advantageously permit the arrangement of several such units in a desired electrical interconnection. However, these semiconductor units do have some drawbacks. For example, the mounting of the disc or wafer of insulating material and of the associated contact plates which are provided with semiconductor chips require undesirable expenditures for fastening by means of a soldering process. Additionally, the metal base plate, which is intended to firmly rest on the surface of a cooling component, is not planar to the desired degree because unavoidable mechanical stresses occur already during the manufacture of the semiconductor units, which stresses are the result of juxtaposed materials having different coefficients of thermal expansion. This impedes the transfer of heat to the cooling component.
Finally, due to the curvature of the base plate, which serves as part of the housing, the connection of the plate with the upper or hollow plastic portion of the housing by means of an adhesive resulted in adhesive layers of different thicknesses. Consequently, when the housing is filled with an insulating mass which surrounds and embeds the components, and the insulating mass hardens, undesirable stresses appear at the line of attachment between the base plate and the plastic housing member which stresses result in further bulging of the base plate.